The present invention relates to a birefringent member cell gap measurement method and instrument for measuring the cell gap of a birefringent member such as a liquid crystal cell.
A liquid crystal cell of twisted nematic structure, which is a type of birefringent member, is employed widely for liquid crystal display units and like apparatus because with such a cell, a high contrast can be readily obtained. Optical properties of the liquid crystal cell of twisted nematic structure depend upon the cell gap (normally less than 10 .mu.m) which is the thickness of a liquid crystal layer constituting the liquid crystal cell and the refractive index anisotropy of a liquid crystal substance constituting the liquid crystal cell. Accordingly, in order to use the liquid crystal cell of twisted nematic structure, it is important to measure the cell gap and the refractive index anisotropy. However, since the refractive index anisotropy depends upon the liquid crystal substance used, it is not easy to vary the refractive index anisotropy. Consequently, it is important to measure the cell gap. Further, since the display uniformity of a liquid crystal display element manufactured using a liquid crystal cell of twisted nematic structure depends upon the uniformity of the cell gap of the liquid crystal cell, in order to obtain a uniform display screen, it is progressively important to measure the uniformity of the cell gap of the liquid crystal cell.
Conventionally, the interferometric method is employed widely as a method of measuring the cell gap of the liquid crystal cell. According to the interferometric method, white light is introduced into a liquid crystal before it is injected with a liquid crystal substance to measure the wavelength dependency (that is, the interference waveform) of the intensity of transmission light or reflection light from the liquid crystal cell. Since the waveform dependency measured at this time has a maximum value and a minimum value depending upon the magnitude of the cell gap of the liquid crystal cell, the cell gap of the liquid crystal cell can be determined from the waveform dependency. In recent years, however, since the method wherein the liquid crystal cell of twisted nematic structure is driven by thin film transistors has become widely used, it has become progressively difficult to accurately determine the cell gap of the liquid crystal cell based on the interferometric method. In particular, since production of thin film transistors for driving the liquid crystal cell on a substrate requires a large number of film formation steps, the liquid crystal cell is formed on the substrate including multi-layered films having different refractive indices from each other. In this instance, since the interference waveform measured includes interference by the multi-layered films in addition to the interference by the cell gap of the liquid crystal cell, it is difficult to accurately determine the cell gap of the liquid crystal cell.
As another method of measuring the cell gap of the liquid crystal cell, there is a method wherein a liquid crystal substance is injected into a liquid crystal cell, following which the cell gap of the liquid crystal cell is measured using a compensator (A. Lien et al., "Cell Gap Measurement of Filled Twisted Nematic Liquid Crystal Display by a Phase Compensation Method," J. Appl. Phys. 69(3), pp. 1304-1309, 1991). In one example of such a case, a Babinet-Soleil compensator is used as the compensator, the liquid crystal cell and the Babinet-Soleil compensator are interposed between a pair of polarization plates, and the amount of transmission light transmitted through the liquid crystal cell and the Babinet-Soleil compensator is measured while the position of a wedge constituting the Babinet-Soleil compensator is mechanically adjusted using a micrometer. The cell gap of the liquid crystal cell is then calculated from the value of a graduation of the micrometer corresponding to the measured amount of transmission light. The cell gap measurement method for a liquid crystal cell which employs a compensator of the type described above may not be influenced by multi-layer films as is the interferometric method described above, but since it requires mechanical adjustment of the Babinet-Soleil compensator and actual reading of the graduation of the micrometer, automation of the measurement method requires a very complicated mechanism, and accordingly, the method is not suitable for automation. Further, since the cell gap measurement method is not suitable for automation, it is also not suitable for measurement of the uniformity of the cell gap of a liquid crystal cell for a liquid crystal display element of twisted nematic structure having a wide area.